Known Issues

Compilation with XLA can greatly improve the performance of your programs, but the TensorFlow interop has a number of known sharp corners.

tf.Variable on a different device

Error message: INVALID_ARGUMENT: Trying to access resource <Variable> (defined @ <Loc>) located in device CPU:0 from device GPU:0

XLA cluster runs on exactly one device, and it can not read or write to tf.Variable located on a different device. Usually this error message indicates that the variable was not placed on the right device to begin with. The error message should precisely specify the location of the offending variable.

NOTE: tf.Variable of type int32 are always placed on a host, and can not be placed on a GPU. As a workaround, int64 can be used.

TensorArray TF/XLA interconversion is not supported

Error message: Support for TensorList crossing the XLA/TF boundary is not implemented.

XLA supports tf.TensorArray. However, the interconversion between TF and XLA representations is not implemented yet. This error often arises when the TensorArray is used inside the compiled block, but the derivative is taken outside.

Workaround: compile the outermost scope which is taking the derivative.

TensorFlow while loops need to be bounded (or have backprop disabled)

Error message: XLA compilation requires a fixed tensor list size. Set the max number of elements. This could also happen if you're using a TensorArray in a while loop that does not have its maximum_iteration set, you can fix this by setting maximum_iteration to a suitable value.

TF while loops created using tf.while_loop support backpropagation by accumulating all intermediate results in a TensorArray, but XLA only supports bounded TensorArrays.

Workaround: all compiled while loops need to either have maximum_iterations parameter set to a constant value known at compile time, or backpropagation disabled using back_prop=False.

Dynamic tf.TensorArray is not supported

Writes into tf.TensorArray(..., dynamic_size=True) are not compilable with XLA, as such writes require an unknown number of reallocations when the array exceeds the original bound.

Workaround: provide a statically known bound to your arrays.

Random number generation ignores TF seed

XLA currently ignores TF seeds to random operations. This affects stateful TF random operations, such as tf.random.normal, or tf.nn.dropout. XLA will behave as if the compilation was seeded with a new unique seed at each run within the same process (the first run of the process will always yield the same result).

Workaround: use the recommended RNGs such as tf.random.stateless_uniform or the tf.random.Generator directly.

Must-be-constant inputs which are functions of induction variables are not supported

Error Message: XLA compilation requires that operator arguments that represent shapes or dimensions be evaluated to concrete values at compile time. This error means that a shape or dimension argument could not be evaluated at compile time, usually because the value of the argument depends on a parameter to the computation, on a variable, or on a stateful operation such as a random number generator.

XLA requires certain values to be known at compile time, such as reduction axis of a reduce operation, or transposition dimensions. Consider the case when e.g. reduction axis is defined as a function of an induction variable of tf.range: resolving it statically is not possible without unrolling the entire loop, which might not be desired by the user.

Workaround: Unroll loops, e.g. by converting tf.range into Python range.

NOTE: The error message above is not unique to this issue, and can arise due to other limitations or bugs.